Learn Italian: Per quanto io usi il repellente, non riesco a scacciarli. - No matter how much I use the repellent, I cannot drive them away.

The conjunction per quanto (meaning "no matter how much" or "however much") introduces a concessive clause and requires the subjunctive. Since the action is happening in the present, it takes the present subjunctive usi.

Is it necessary to include the subject pronoun io before usi?

It is highly recommended for clarity. In the present subjunctive, the conjugations for io, tu, and lui/lei are identical (they are all usi). Adding io makes it immediately clear who is using the repellent.

Why translate "I cannot" as non riesco a instead of non posso?

Riuscire a means "to manage to" or "to succeed in". Using non riesco a emphasizes that you are trying to drive them away but failing despite your efforts. Non posso (from potere) would normally imply a lack of permission or a strict physical impossibility.

How is scacciarli formed?

Scacciarli is a combination of the infinitive scacciare (to drive away) and the direct object pronoun li (them). When attaching a pronoun to an infinitive in Italian, you always drop the final -e of the verb before adding the pronoun.